1. Field of the Invention
This invention relates to novel electromagnetic induction heaters which can heat fluids such as water and steam stably to a predetermined temperature. More specifically, the invention concerns a super-heated steam generator which can heat steam to a temperature of 100.degree. C. or above under normal pressure.
2. Description of the Prior Art
Steam can provide high latent heat or heat of condensation, and therefore it is useful as source of heat. Particularly, steam at 100.degree. C. or above is useful in various fields such as boilers, concentrated air conditioning systems, heating sources for various factory machines and apparatuses, irons and steamers for food. Steam is further used for various other purposes.
Heretofore, steam at 100.degree. C. or above can be obtained in a steam piping provided in a multi-pipe heat exchanger or the like by burning such fuel as petroleum, gas and coal, while at the same time the steam is saturated by application of pressure (of 20 to 60 atmospheres (kg/cm.sup.2), for instance). Alternatively, the steam piping is heated with combustion gas or an electric resistance heater.
However, where petroleum, coal, natural gas, etc. are burned for boilers or the like, fire prevention or like safety means are necessary. In addition, because of very great temperature difference between the heating portion and water or steam that is heated, what is commonly termed "scale" deposits in the heating pipe, reducing the coefficient of heat transfer and eventually resulting in cracks in the pipe. Therefore, it is necessary to carry out scale prevention treatment of water supplied to the boilder in advance by removing bubbles (oxygen removal), using chemical agents or by maintaining alkaline property of water. Moreover, a system is widely practiced in hotels or the like in which steam is produced by burning petroleum, coal, natural gas, etc. and circulated as a source of room heat or the like in the overall building. Such a system, however, is subject to great energy loss and cannot be an efficient system at all times.
Further, where an electric resistance heater is provided in water, water is heated to a temperature far higher than 100.degree. C., i.e., its boiling point, in the neighborhood of the heat source. Therefore, if a heater without a sufficient boundary surface heat transfer area is used, various troubles are produced.
Further, since an electric resistance heater, like the burning of gas, produces extraordinary temperature difference between the heating source and the water, inorganic and organic components contained in water are adsorbed to and accumulated on the heater surface and act as heat insulator, thus reducing the heat conductivity and retarding the boiling of the water. At the same time, heat radiation from the heater deteriorates, eventually leading to heater lead breakage. To avoid this accident, the heater for heating water is provided with great surface area and accommodated in the full space of the water tank, thus presenting the problems of cumbersomeness of heater exchange and also reliability problems.
Further, washing the heating element, which is required due to attachment of filth, is very timeconsuming.
Further, it is difficult to obtain accurate steam temperature control, which is has heretofore been basically impossible to improve.
Further, in the above case it is necessary to provide an absolute pressure of about 16 kg/cm.sup.2 for obtaining saturated steam at 200.degree. C., to provide an absolute pressure of about 41 kg/cm.sup.2 for obtain steam at 250.degree. C. and to provide an absolute pressure of about 90 kg/cm.sup.2 for obtaining steam at 300.degree. C. This means that the prior art steam generator inconveniently requires the use of a pressure-bearing vessel.